Synthesis and biological activity of a potent and orally bioavailable SCD inhibitor (MF-438)

Article abstract of DOI:10.1016/j.bmcl.2009.11.111

A series of stearoyl-CoA desaturase 1 (SCD1) inhibitors were developed. Investigations of enzyme potency and metabolism led to the identification of the thiadiazole-pyridazine derivative MF-438 as a potent SCD1 inhibitor. MF-438 exhibits good pharmacokine

Full text of DOI:10.1016/j.bmcl.2009.11.111

Bioorganic & Medicinal Chemistry Letters 20 (2010) 499–502
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological activity of a potent and orally bioavailable SCD
inhibitor (MF-438)
Serge Léger, W. Cameron Black, Denis Deschenes, Sarah Dolman, Jean-Pierre Falgueyret, Marc Gagnon,
Sébastien Guiral, Zheng Huang, Jocelyne Guay, Yves Leblanc, Chun-Sing Li, Frédéric Massé, Renata Oballa ,
*
Lei Zhang
Merck Frosst Centre for Therapeutic Research, PO Box 1005, Pointe-Claire-Dorval, Québec, Canada H9R 4P8
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of stearoyl-CoA desaturase 1 (SCD1) inhibitors were developed. Investigations of enzyme potency
and metabolism led to the identification of the thiadiazole–pyridazine derivative MF-438 as a potent
SCD1 inhibitor. MF-438 exhibits good pharmacokinetics and metabolic stability, thereby serving as a
valuable tool for further understanding the role of SCD inhibition in biological and pharmacological mod-
els of diseases related to metabolic disorders.
Received 1 October 2009
Revised 18 November 2009
Accepted 20 November 2009
Available online 26 November 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
SCD inhibitor
Stearoyl-CoA
Pyridazine
Thiadiazole
Bioavailable
Obesity, fatty liver disease, type 2 diabetes and atherosclerosis
are occurring with increasing frequency, a phenomenon generally
attributedto physicalinactivity and nutrient oversupply. Dysregula-
tion of lipid metabolism has been identified as a critical contributor
to the pathogenesis of these related disorders. Stearoyl-CoA desatur-
ase (SCD) is a long chain fatty acyl-CoA specific desaturase with a
putative di-iron-oxo-containing active site.1 SCD catalyzes the for-
pound 1) were found to be extensively metabolized on the alkyl
chain of the amide.13 To provide compounds suitable for in vivo
models, it is necessary to transform the secondary amide into a met-
abolically more resistant chemical entity. As depicted in Table 1,
removalof thealkylchain (2) resultedin a modest potencyloss when
tested in a SCD1-induced rat liver microsomal assay.14
Potency can be improved by changing the linker between the
piperazine and the trifluoromethylphenyl moiety. Replacement of
the amide linker with an ether (3) improves the SCD1 potency by
more than 20-fold in the rat microsomal assay. Additionally, the po-
tency gain observed with the ether linker is retained when the met-
abolically labile alkyl chain of the secondary amide is removed (4).
Having metabolically stabilized two potentially vulnerable
areas of the molecule, an evaluation of the pharmacokinetic profile
in rats was performed with the primary amide 4. Poor pharmaco-
mation of a cis-double bond at the D9-position of the preferred sub-
strate stearoyl-CoA. The resulting oleoyl-CoA is a major fuel
intermediate for b-oxidation and a key substrate in triglyceride, cho-
lesterol ester, phospholipid and lipid signaling molecule production.
Four SCD isoformshave been characterizedin rodents and two in hu-
mans.2 SCD1, with ꢀ85% identity across species, is the major isoform
found in lipogenic tissues including liver and adipose. Emerging evi-
dence supports the hypothesis that elevated SCD1 activity is a key
player in the development of obesity, fatty liver, insulin resistance
and related metabolic disorders.3–5 In humans, elevated SCD activity
is positively associated with a high body mass index, hyperinsuline-
mia, hypertriglyceridemia and liver steatosis.5–7 Therefore, SCD1
inhibition may represent a novel treatment for obesity, diabetes,
atherosclerosis and related metabolic disorders and to date, many
groups have published on various SCD inhibitor scaffolds.8–12
Small molecule SCD inhibitors recently described in the
literature, which are derived from pyridazine amides, (e.g., com-
kinetic properties were still observed (F = 10%, t = 0.54 h). This
½
was attributed to hydrolysis of the primary amide to form the inac-
tive acid metabolite 5. This metabolite was shown to be the major
circulating species with an AUC for compound 5 being 200-fold
greater than the AUC of compound 4 (Fig. 1).
From this pharmacokinetic study, it became clear that the
amide required replacement with a metabolically more robust
moiety. We explored the use of an imidazole as an amide surro-
gate, a small heterocycle providing the required polarity. Replace-
ment of the primary amide of 4 with an imidazole afforded 8p
(Table 2), which was demonstrated to be metabolically robust as
determined by pharmacokinetic analysis in rats (F = 53%,
* Corresponding author.
E-mail address: renata_oballa@merck.com (R. Oballa).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.11.111

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S. Léger et al. / Bioorg. Med. Chem. Lett. 20 (2010) 499–502
Table 1
Table 2
SAR summary of amide-pyridazine compounds
SAR summary of substitution on the phenoxy moiety in the imidazole–pyridazine
series
rSCD1 IC50 (nM)
322
N
N
N
O Ar
O
O
N N
N
N
CF3
1
2
3
4
5
N
H
N N
N
Compound
Ar
rSCD1 IC50 (nM)
8a
8b
8c
8d
8e
8f
8g
8h
8i
Phenyl
2-Tolyl
3-Tolyl
4-Tolyl
2-Ethylphenyl
3-Ethylphenyl
4-Indanyl
1890
39
251
527
4
38
10
62
162
543
236
34
5
23
4
19
371
O
O
N
CF3
692
H2N
N N
O
N
O
CF3
5-Indanyl
4-Indolyl
14
N
H
N N
N
8j
2-Methoxyphenyl
2-Cyanophenyl
2-Acetylphenyl
2-Bromophenyl
3-Bromophenyl
2-CF3-phenyl
3-CF3-phenyl
4-CF3-phenyl
8a
8b
8c
8d
8e
8f
O
O
CF3
16
H2N
N N
O
N
8g
O
CF3
ꢀ10,000
HO
N N
HN
OH
N
N
N
Cl
N
N
OH
Compound 4 dosed PO (AUC = 0.9 uM*h)
Compound 4 dosed IV (AUC = 1.9 uM*h)
N N
6
N N
a
35
30
25
20
15
10
5
7
Acid 5 observed when
amide 4 was dosed PO (AUC = 179 uM*h)
HO
X, Y
X, Y
N
N
N
O
N N
b
8
Scheme 1. Reagents and conditions: (a) TBAI, K2CO3, 1,4-dioxane, reflux 16 h. (b)
DEAD, PPh3, THF, room temperature, 16 h.
phenyl ethers (8a–r) required to establish the SAR of the phenoxy
portion.
The potency of this class of inhibitors is strongly influenced by
the substitution pattern on the phenyl ether portion (Table 2). This
is clearly illustrated by comparing the unsubstituted phenyl ether
(8a), with a compound containing even a simple methyl substitu-
ent (8b). In general ortho substituted aryl ethers were preferred
over meta or para substituted analogs, regardless of the nature of
the substituent. In addition, increasing the bulkiness of the substi-
tuent is also desirable in term of intrinsic potency. Bicyclic systems
are also tolerated on the aromatic portion of these compounds,
although introduction of polar heteroatoms appears to compro-
mise their potency (8g vs 8i). The introduction of heteroatoms on
monocyclic systems was also evaluated: electron-donating (8j) or
electron-withdrawing (8k) groups improve the potency relative
to the unsubstituted phenyl but were less desirable compared to
lipophilic groups. Halides (8n) and haloalkyls (8p) were found to
be the preferred substituents; generally this type of substitution
also affords metabolic robustness.
0
0
1
2
3
4
5
6
Time (h)
Figure 1. Plasma concentration versus time profile of compounds
4
(closed
squares) and 5 (opened squares) when compound 4 is dosed in rats at 10 mg/kg
PO and 2 mg/kg IV (closed circles). Concentrations are the average of two animals
per time point.
t
½
= 1.2 h, data not shown). Having addressed the metabolic issues
with this analog, we sought to explore the SAR around the aryl ring
of the phenoxy-piperidine moiety. A series of phenyl ethers were
rapidly accessed from the 4-piperidinol derivative 7, prepared via
a nucleophilic aromatic substitution from the commercially avail-
able 3-chloro-6-(1H-imidazol-1-yl)pyridazine 6 (Scheme 1). This
was followed by a Mitsunobu reaction, which afforded a series of
Following this analysis, compound 8p was for further profiled in
a variety of in vitro and in vivo assays. Unfortunately, it was found
that 8p inhibits cytochrome P450 enzymes (IC50
of 0.4 lM vs
CYP3A4 and 0.3 lM vs CYP2D6). Based on literature precedent
showing that imidazoles are often responsible for CYP inhibition,15
the replacement of the imidazole heterocycle was investigated.

S. Léger et al. / Bioorg. Med. Chem. Lett. 20 (2010) 499–502
501
Ring systems of different size and polarity were evaluated for
F3C
their ability to retain potency while reducing the potential for
CYP inhibition. When bicyclic systems like benzothiazoles or benz-
imidazoles were used to replace the imidazole, the resultant loss in
SCD potency prompted us to focus on small heterocycles (data not
shown). Several small heterocycles were evaluated; some were not
pursued due to potential metabolic stability issues like the 1,2,4-
oxadiazoles,16 while others did not provide the required potency
profile. Table 3 illustrates a series of 1,3,4-oxa and thia-diazoles
which demonstrated the targeted profile. Substitution at the 5-po-
sition was evaluated in order to optimize potency. Comparison of
the unsubstituted oxadiazole 9a with other 5-alkyl analogs reveals
that substitution is beneficial for SCD potency.
MeO2C
N
N
11
12
O
N N
a, b
F3C
O
O
AcN N
H H
N N
c
F3C
Me
S
N
N
O
However, comparison of analogs b with d and e, reveals that the
size of the aliphatic substituent cannot be large. The potency is not
significantly affected by addition of polarity in this area of the mol-
ecule and a hydroxymethyl group is well tolerated (9g).
N
N N
MF-438
Scheme 2. Reagents and conditions: (a) hydrazine hydrate, MeOH, reflux 2 h; (b)
AcCl, Hunig’s base, CH2Cl2, 1 h; (c) P2S5, xylene, 160 °C, 16 h.
In the thiadiazole series the methyl analog 10b, or MF-438, was
identified as the most promising compound in this series. MF-438
is readily accessible from the methyl ester analog 11 of acid 516 in
three steps. First, conversion of the ester to a hydrazide followed
by acetylation afforded compound 12. Direct formation of 12 from
ester 11 using acethydrazide consistently gave unsatisfactory
yields and a stepwise approach was found to be more efficient. Fi-
nally the formation of the thiazole was achieved by refluxing com-
pound 12 with phosphorus pentasulfide in a high boiling, non-
polar solvent17 (Scheme 2).
MF-438 has an overall good pharmacokinetic profile in rodents.
Oral bioavailability was 73% in mice and 38% in rats with half-lives
of 6.4 and 6.0 h, respectively. A circulating metabolite resulting
from the oxidation of the methyl of the thiazole to the hydroxy-
methyl (10g) was observed in both species at all time points, but
was present at only approximately 10% of the concentration of
the parent compound.
Upon chronic dosing in animal models, this compound dis-
played adverse effects similar to other SCD inhibitors recently re-
ported.13,19 After approximately one week of qd dosing with MF-
438 at 5 mg/kg in DIO mice, the first symptoms of alopecia and
partial eye closure began to appear. The severity and the time at
which these adverse effects were observed were directly related
to the dose being administered. Similar adverse effect patterns
were observed in other rodent models such as the obese diabetic
Zucker rats.20 Importantly, these adverse effects were shown to
be reversible upon cessation of treatment.
In conclusion we have described the SAR which led to the iden-
tification of MF-438, a potent SCD inhibitor. The in vivo metabolic
and pharmacokinetic profiles of MF-438 are greatly improved over
previously described amide-based inhibitors, thus enabling this
compound to serve as a valuable tool for in vivo assessment of
SCD inhibition.
The CYP inhibition issues encountered with the imidazole com-
pound 8p was resolved with the thiadiazole analog. MF-438 does
not inhibit either CYP3A4 or CYP2D6 at concentrations up to 30 lM.
References and notes
The long half-lives observed in rodents make this compound
suitable for once-daily dosing and as such MF-438 became an
excellent tool for in vivo assessment of SCD inhibition. Indeed, this
compound was found to be very potent in vivo in a mouse liver PD
assay which measures SCD inhibition in the liver of mice on a high
carbohydrate diet.18 The studied compound was administered PO
followed by an IV administration of a 14C labeled stearic acid tracer,
1 h later. After an additional 2 h, mouse livers were harvested and
analyzed for their lipid content. Inhibition of SCD activity in the li-
ver was determined by comparing the conversion of 14C-stearic
acid to 14C-oleic acid of treated animals versus a vehicle control
group. MF-438 exhibited an ED50 between 1 and 3 mg/kg in this
mouse model.
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X = O
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—
2.3 MF-438
8.8
37
710
6.1
3.2
a
b
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d
e
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Me
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